The present invention relates to a spray-coating method and spray-coater.
Coating paper with a pigment slurry is performed by applying a smooth layer of coating to the surface of a paper web, after which excess water is removed from the web by means of dryers. Over the times, the application and leveling has been carried out with the help of different apparatuses, whose development has been directed by the increasing requirements toward higher quality and productivity, as well as faster web speeds in papermaking machines.
One technique suitable for use in coating a paper web is the so-called spray-coating method that is described, e.g., in the international patent application publication WO 97/13035. In the spray-coating technique the coating mix is applied to the surface of paper web by spraying nozzles in which the coating mix is atomized into small droplets prior to spraying onto the moving paper web. The coat quality can be controlled by adjusting the distance between the spraying nozzle and the web, the spray-jet velocity and the mass rate of spraying. Generally, there must be disposed a number of spraying nozzles in the cross-machine direction to the web travel, because the coverage of the spraying jet emitted by a single nozzle is usually narrower than the width of the web being coated. Typically, there is disposed a number of nozzle rows across the web, perpendicular to the web travel, since a single row of nozzles extending across the web usually fails to provide a coating of a sufficiently high quality. The coating mist not adhering to the web during the coating process can be recovered for reuse. To prevent the coating mist from escaping to the environment, the spraying nozzles are generally located in a closed space such as an enclosing hood.
After the application of the coating, the paper web must be dried in a noncontacting manner because the wet coating is sensitive to mechanical contacts. Generally, the noncontacting drying is carried out using infrared or air-impingement dryers. Cylinder dryers can be used only after the coating has attained a sufficiently low moisture content and will not any more adhere to the cylinder.
Conventionally, the coating is applied to the surface of the paper web in several steps, and the paper web may also be subjected to further drying between the successive application steps. This kind of a coater station involving a plurality of applicator/dryer devices requires a lot of space which complicates its integration with a paper machine. One of the problems to be solved in the spray-coating of paper sheet is how to apply the coating emitted from the spraying nozzles so as to attain a uniform coat weight and quality at both the center and the edges of the paper web. Usually, the applied coating layer at the edge areas of the coated paper web tend to be thinner than at the center area of the web, whereby it will be problematic to reach a smooth coat profile up to the web edges. Because the gap between the moving web and the enclosing hood is difficult to seal, it is also impossible to fully prevent the coating mist from escaping to the environment from the hood enclosing the spraying-nozzle assemblies. The spreading of the coating mist to the exterior of the enclosing hood can be reduced by using vacuum suction to remove the coating mist from the interior of the hood and then to separate the coating solids from the recovered coating mist for reuse. However, these kinds of arrangements need a separate filter for recovering coating solids from the mist. The coating mist filling the interior of the enclosing hood also tends to readily adhere to and dry on the surfaces such as the spraying nozzles housed in the interior of the hood.
It is an object of the present invention to provide a novel type of spray-application method and apparatus capable of overcoming the above-described problems.
The arrangement according to the invention replaces a roll or wire conventionally used for supporting the web by air jets directable to the surface of the web. Additionally, air jets may be used for drying the coating applied to the web surface. Furthermore, edge deckles are disposed close to the web edge between the spraying nozzles and the paper web that serve to prevent coating sprays discharged from nozzles close to the web edge from reaching the web surface at the edges. With the help of an exhaust nozzle connected to the hood enclosing the spraying nozzles, the interior of the hood is brought to a partial vacuum relative to the ambient pressure thus preventing the coating mist from escaping to the exterior of the hood. Another nozzle connected to the enclosing hood can be used for feeding air or steam into the hood in order to bring the interior temperature and humidity to a level that prevents the coating mist from depositing and drying onto the interior surfaces of the hood. Finally, the interior walls of the hood can be made coolable, whereby the coating mist will condense on the cooled surfaces.
The invention offers significant benefits.
Air jets employed in the noncontacting supporting of a paper web may replace dryer devices, whereby the dryer section may be made smaller in size and more readily integratable in the layout of the papermaking machine. With the help of the edge deckles according to the invention, the coat profile at the web edges can be made uniform and the cross-machine width of the applied coat can be easily varied. Due to the partial vacuum in the interior of the hood enclosing the nozzles, the coating mist cannot escape from the hood so as to soil equipment exterior to the hood. Furthermore, the adherence and drying of the coating onto the interior parts and surfaces of the hood is reduced thus relaxing the need for their frequent cleaning. Devices specifically intended to the separation of coating solids from the coating mist can be omitted, because of the condensation of solids from the coating mist onto the cooled interior surfaces of the enclosing hood, wherefrom the solids can be recovered for reuse in the coating mix circulation.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.